Automatic kinescope bias system with AC coupled video output stage

Abstract

A video processing and display system includes an image displaying kinescope, an AC coupled kinescope driver stage, and a system for automatically controlling the kinescope bias. During image blanking intervals the kinescope grid electrode is excited to induce a cathode output pulse related to the magnitude of the kinescope black current level. A DC restoration circuit responds to the induced pulse and provides to the kinescope a bias control signal related to the magnitude of the induced pulse, for maintaining a desired kinescope black current bias.

What is claimed is: 
     
       1. In a video signal processing system including an image display device for displaying video information in response to a video signal applied to an intensity control assembly thereof including an intensity control electrode, apparatus comprising: a driver amplifier stage;   means for AC coupling video signals from an output of said driver amplifier;   a current path for coupling video signals from said AC coupling means to said intensity control assembly; and   bias control means coupled to said display device for automatically maintaining a desired bias condition for said display device, wherein said bias control means comprises   means for generating a signal with a magnitude representative of the magnitude of the black current conducted by said display device during image blanking intervals;   DC restoration means responsive to said representative signal for providing an output biasing signal related to the magnitude of said representative signal; and   means for coupling said biasing signal to said intensity control assembly to maintain said desired bias condition.   
     
     
       2. Apparatus according to claim 1, wherein said current path couples video signals to said intensity control electrode; and   said biasing signal is coupled to said intensity control electrode via said current path.   
     
     
       3. Apparatus according to claim 2, wherein said image display device comprises a kinescope and said intensity control assembly comprises a cathode intensity control electrode and an associated grid electrode; and   said generating means comprises means for forward biasing said intensity control assembly during image blanking intervals to produce a cathode output signal corresponding to said black current representative signal.   
     
     
       4. Apparatus according to claim 3, wherein said generating means provides a forward biasing excitation signal to said grid electrode to produce said cathode output signal.   
     
     
       5. Apparatus according to claim 1, wherein said driver amplifier stage includes an output circuit with first and second series resistances coupled between a signal output of said amplifier stage and an operating potential, said first resistance being connected nearer to said signal output and said second resistance being connected nearer to said operating potential;   said generating means comprises means for exciting said intensity control assembly during image blanking intervals to induce an output signal from said intensity control assembly corresponding to said black current representative signal; and   a signal input of said DC restoration means is coupled to a junction between said first and second resistances for sensing the magnitude of said induced output signal.   
     
     
       6. Apparatus according to claim 5, wherein the value of said first resistance is significantly greater than the value of said second resistance.   
     
     
       7. In a video signal processing system including an image display device for displaying video information in response to a video signal applied to an intensity control assembly thereof including an intensity control electrode, apparatus comprising: a driver amplifier stage with a video signal input and a video signal output circuit;   means for AC coupling video signals from said output circuit of said driver amplifier;   a current path for coupling video signals from said AC coupling means to said intensity control assembly; and   bias control means coupled to said display device for automatically maintaining a desired bias condition for said display device, wherein said bias control means comprises   means for applying a forward biasing excitation signal to said intensity control assembly during image blanking intervals to induce an output signal from said intensity control assembly representative of the magnitude of black current conducted by said display device;   means coupled to said output circuit of said driver amplifier for sensing said induced output signal at said output circuit;   means responsive to said sensed induced output signal for developing a biasing signal with a magnitude related to the magnitude of said induced output signal; and   means for coupling said biasing signal to said intensity control assembly to maintain said desired bias condition.   
     
     
       8. Apparatus according to claim 7, wherein said image display device is a kinescope;   said intensity control assembly includes a cathode electrode and an associated grid electrode;   said AC coupling means includes a capacitance coupled between said output circuit of said driver amplifier and said cathode electrode;   said excitation signal is applied to said grid electrode for inducing a cathode output signal corresponding to said black current representative signal; and   a DC restoration circuit responds to said induced cathode output signal for providing said biasing signal with a magnitude related to the magnitude of said induced cathode output signal.   
     
     
       9. In a video processing system including an image display device for displaying video information in response to video signals applied to an intensity control assembly thereof, apparatus comprising: a driver amplifier stage;   means comprising an AC coupled current path for conveying video signals from an output circuit of said driver amplifier to said intensity control assembly of said display device; and   bias control means coupled to said display device for automatically maintaining a desired bias condition therefor, said bias control means including   means for exciting said intensity control assembly during image blanking intervals to produce an output signal with a magnitude representative of the magnitude of black current conducted by said display device during image blanking intervals;   means for sensing said representative output signal at a point in said AC coupled current path;   means for developing a bias control signal related to the magnitude of said representative signal; and   means for applying said bias control signal to said intensity control assembly.   
     
     
       10. Apparatus according to claim 9, wherein said AC coupled current path comprises a coupling capacitor with an input coupled to said output of said driver stage, and an output coupled to said intensity control assembly;   said representative signal is sensed at a point prior to said capacitor input; and   said bias control signal is applied to a point after said capacitor output.